Method to produce equal sized features in microlithography

ABSTRACT

A microlithography mask for producing equal size features in a substrate. A first region exposes a first portion of the substrate corresponding to a first feature that is to be formed on the substrate. At least one compensating region in the vicinity of the first region partially exposes the first portion of the substrate and a second portion of the substrate corresponding to a second feature, wherein the second feature is to be removed from the substrate.

FIELD OF THE INVENTION

[0001] The present invention relates to manufacture of integratedcircuit devices using microlithography techniques. In particular, thepresent invention is directed to a mask and method for creating isolatedfeatures that have similar physical characteristics to similar featuresthat are not isolated.

BACKGROUND OF INVENTION

[0002] Very large scale integrated circuit devices typically aremanufactured on a substrate, such as a silicon wafer, by a sequence ofmaterial additions, such as low pressure chemical vapor depositions,sputtering operations, among others; material removals, such as wetetches, reactive ion etches, among others; and material modifications,such as oxidations, ion implants, among others. Typically, thesephysical and chemical operations interact with the entire substrate. Forexample, if a substrate is placed into an acid bath, the entire surfaceof the substrate will be etched away. In order to build very smallelectrically active devices on a substrate, the impact of theseoperations has to be confined to small, well-defined, regions.

[0003] Lithography in the context of VLSI manufacturing includes theprocess of patterning openings in photosensitive polymers, sometimesreferred to as “photoresists” or “resists”, which define small areas inwhich substrate material is modified by a specific operation in asequence of processing steps.

[0004] The radiation preferably causes desired photochemical reactionsto occur within the photoresist. Preferably, the photochemical reactionsalter the solubility characteristics of the photoresist, therebyallowing removal of certain portions of the photoresist. Photoresistscan be negative photoresist or positive photoresist materials.

[0005] A negative photoresist material is one which is capable ofpolymerizing and being rendered insoluble upon exposure to radiation.Accordingly, when employing a negative photoresist material, thephotoresist is selectively exposed to radiation, causing polymerizationto occur above those regions of the substrate which are intended to beprotected during a subsequent operation. The unexposed portions of thephotoresist are removed by a solvent which is inert to the polymerizedportion of the photoresist. Such a solvent may be an aqueous solventsolution.

[0006] Positive photoresist material is a material that, upon exposureto radiation, is capable of being rendered soluble in a solvent in whichthe unexposed resist is not soluble. Accordingly, when applying apositive photoresist material the photoresist is selectively exposed toradiation, causing the reaction to occur above those portions of thesubstrate which are not intended to be protected during the subsequentprocessing period. The exposed portions of the photoresist are removedby a solvent which is not capable of dissolving the exposed portion ofthe resist. Such a solvent may be an aqueous solvent solution.

[0007] Selectively removing certain parts of the photoresist allows forthe protection of certain areas of the substrate while exposing otherareas. The remaining portions of the photoresist may be used as a maskor stencil for processing the underlying substrate. For example, theopenings in the mask may allow diffusion of desired impurities throughthe openings into the semiconductor substrate. Other processes are knownfor forming devices on a substrate.

[0008] The manufacturing of VLSI chips typically involves the repeatedpatterning of photoresists, followed by etch, implant, deposition, orother operation, and ending with the removal of the exposed photoresistto make way for the new photoresist to be applied for another iterationof this process sequence.

[0009] Devices such as those described above, may be formed byintroduction of a suitable impurity into a wafer of a semiconductor toform suitably doped regions therein. In order to provide distinct P or Nregions, which are necessary for the proper operation of the device,introduction of impurities should occur through only a limited portionof the substrate. Usually, this is accomplished by masking the substratewith a diffusion resistant material, which is formed into a protectivemask to prevent diffusion through selected areas of the substrate.

[0010] Basic lithography systems typically include a source of light,typically not visible light, a stencil or photomask including a patternto be transferred to a substrate, a collection of lenses, and a meansfor aligning existing patterns on the substrate with patterns on themask or stencil.

[0011] Conventional photomasks typically consist of chromium patterns ona quartz plate, allowing light to pass wherever the chromium has beenremoved from the mask. Light of a specific wavelength is projectedthrough the mask onto the photoresist coated substrate, exposing thephotoresist wherever chromium has been removed from the mask permittinglight to pass through the mask. Exposing the resist to light of theappropriate wavelength causes modifications in the molecular structureof the resist polymers, which permits the use of developer to dissolveand remove the resist in the exposed areas. Resists that act as justdescribed are known as “positive” resists. On the other hand, negativeresist systems permit only unexposed areas to be removed by thedeveloper.

[0012] Photomasks, when illuminated, can be pictured as an array ofindividual, infinitely small light sources that can be either turned on,such as areas not covered by chromium or other material, or turned off,such as areas covered by chrome or other material. If the amplitude ofthe electric field vector that describes the light radiated by theseindividual light sources is mapped across a cross-section of the mask, astep function will be plotted reflecting the two possible states thateach point of the mask can be found, either light on or light off.

[0013] Conventional photomasks are commonly referred to as “chrome onglass” (COG) binary masks, due to the binary nature of the imageamplitude. The perfectly square step function of binary masks actuallyexists only in scalar theory and typically only in the level of theexact mask plane. Any distance away from the mask, such as at thesubstrate plane, diffraction will cause images to exhibit a finite imageslope. At small dimensions, that is, when the size and spacing of theimages to be printed are small relative to the wavelength and inverse ofthe numerical aperture, the electric field vectors of adjacent imageswill interact and add constructively.

[0014] Therefore, not only is diffraction a phenomenon that must beaddressed when dealing with very small images, interference must also beaddressed. The resulting light intensity curve between features is notcompletely dark, as a result of the diffraction and interferencephenomenon. Rather, the light intensity curve exhibits significantamounts of light intensity created by the interaction of adjacentfeatures.

[0015] The resolution of an exposure system is limited by the contrastof the projected image, that is, the intensity difference betweenadjacent light and dark features. An increase in the light intensity innominally dark regions will eventually cause adjacent features to printas one combined structure rather than as discrete images.

[0016] In an effort to increase the capability of electronic devices,the number of circuit features included on, for example, a semiconductorchip, has greatly increased. When using a process such as that describedabove for forming devices on, for instance, a semiconductor substrate,increasing the capability and, therefore, the number of devices on asubstrate requires reducing the size of the devices or circuit features.One way in which the size of the circuit features created on thesubstrate has been reduced is to employ mask structures having smalleropenings.

[0017] Such smaller openings treat smaller portions of the substrate,thereby creating smaller structures in the photoresist. In order toproduce smaller structures in the photoresist, shorter wavelengthultraviolet radiation is also used in conjunction with the mask to imagethe photoresist. Such shorter wavelengths of radiation have also beenparticularly effective at curing or hardening photoresist materials usedin fabricating the devices.

[0018] The increasingly small and densely packed devices being formed onsemiconductor substrates increases the effects of diffraction, forexample. As discussed above, radiation passing through a mask insemiconductor device manufacture processes behaves as it does in anyother context. Accordingly, as a result of diffraction of the radiation,the radiation may expose areas of the substrate not directly in linewith the transparent area of the mask that the radiation is passingthrough. Diffraction of radiation may result in a feature of an intendedsize being formed of different sizes, as a result of whether the featureis formed in a group of other features or in isolation.

SUMMARY OF INVENTION

[0019] Aspects of the present invention provide a microlithography maskfor producing equal size features in a substrate. A first region of themask is for exposing a first portion of the substrate corresponding to afirst feature that is to be performed on a substrate. The mask alsoincludes at least one compensating region in the vicinity of the firstregion for partial exposing the first feature and it also exposing asecond portion of the substrate corresponding to a second feature,wherein the second feature is to be removed from the substrate.

[0020] Aspects of the present invention also provide a method of formingan isolated image segment having physical characteristics of an opticalproximity affected segment in a semiconductor mask. A portion of a firstphotoresist layer on the substrate corresponding to a first feature thatis to be formed in the first photoresist layer is exposed. At least onecompensating portion of the first photoresist layer adjacent the firstregion is exposed such that exposing the compensating portion at leastpartially exposes the first feature. The at least one compensatingfeature is subsequently removed.

[0021] Still other aspects, objects, and advantages of the presentinvention will become readily apparent by those skilled in the art fromthe following detailed description, wherein it is shown and describedonly the preferred embodiments of the invention, simply by way ofillustration of the best mode contemplated of carrying out theinvention. As will be realized, the invention is capable of other anddifferent embodiments, and its several details are capable ofmodifications in various obvious respects, without departing from theinvention. Accordingly, the drawings and description are to be regardedas illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above-mentioned objects and advantages of the presentinvention will be more clearly understood when considered in conjunctionwith the accompanying drawings, in which:

[0023]FIG. 1a and FIG. 1b represent a feature formed in isolation on asubstrate and its width and a feature formed in a group of features andits width, respectively;

[0024]FIGS. 2a-c represent various stages of a process according to thepresent invention for forming a feature of substantially the same sizein isolation or in a group of features;

[0025]FIGS. 3a and 3 b illustrate a method to quantify diffractionaffects of optical CD metrology tool; and

[0026]FIGS. 4a and 4 b illustrate a method to measure the true nested toisolated CD delta.

DETAILED DESCRIPTION OF THE INVENTION

[0027] As mentioned above, as result of diffraction of the radiation, afeature to be formed in, for example, photoresist, may have differentcharacteristics when formed isolated from other features on a substrateas compared to when the same feature is formed closely grouped withother features. The present invention provides a method and masks toproduce equal size features, regardless of where the features are formedin isolation or in close proximity to other features.

[0028] As stated above, radiation used to expose photoresist on asubstrate during semiconductor device manufacturer is subject to thesame physical laws as radiation in any other environment. Therefore, asthe radiation passes through transparent areas of microlithographymasks, the radiation is subject to diffraction effects. Therefore, theisolated feature typically will not have the same size as the featureformed in close proximity to other features.

[0029]FIG. 1a and FIG. 1b illustrate, respectively, a feature formed inisolation and a feature formed in a group of features. The feature 1illustrated in FIG. 1a and the feature 3 illustrated in FIG. 1b areformed from the same size opaque lines in the microlithography mask.Feature 1 has a width w_(i), while feature 3 has a width w_(n). Feature3 is formed with features 5 and 7 in close proximity thereto.

[0030] As features 3, 5 and 7 are exposed, due to diffraction effects,feature 3 will be affected by adjacent features 5 and 7. As a result,when features are formed in a group as illustrated in FIG. 1b, thefeatures typically will have a slightly smaller size than the opaqueportion of the mask designed to create the feature on the substrate.This is illustrated in FIG. 1a and FIG. 1b by the difference in widthbetween the features 1 and the feature 3.

[0031] In addition to optical diffraction or optical proximity effects,etch loading effects also cause nested features to be different thanisolated features. For example, IC fabrication resist features aretypically transferred into substrates such as silicon by using etchprocesses such as reactive ion etching. Due to different etch loading,isolated features and nested features are etched differently especiallywhen the features have a large aspect ratio (the ratio between etchdepth and feature width). The present invention addresses thenested-to-isolated linewidth delta caused by both optical proximity andchemical processes, such as etch and resist develop.

[0032] The problem is that if it is desired that features 1 and 3 havethe same width, if the features are produced according to known methodsutilizing known microlithography masks, w_(i) will not equal w_(n). Thisdisparity in feature sizes is particularly important in relation todevice speed sorts.

[0033] Although the above discussion relates particularly to issuesrelated to wafer printing, another important component inmicrolithography is mask printing. Masks are subject to similarnested-to-isolated linewidth variation as in wafer printing. A mask istypically printed either by laser beam or electron beam lithography. Theexposed resist is then developed and finally the resist pattern istransferred into final mask material by etching. Proximity effects oflaser beam or electron beam printing and etch loading effect all causenested-to-isolated linewidth delta on the mask. The present invention isuseful to these and other issues related to the mask level.Additionally, the current invention addresses mask critical dimension(CD) metrology issues. The present invention also includes methods andmasks to characterize CD metrology tools used in mask fabrication.

[0034] The present invention provides a natural way of making allfeatures the same way rather than utilizing a separate method andapparatus for creating isolated features. According to the presentinvention, a microlithography mask is utilized that includes a firstregion for exposing a first portion of a substrate corresponding to afirst feature that is to be formed on the substrate. The mask alsoincludes at least one compensating region in the vicinity of the firstregion for partially exposing the first portion of the substrate and thesecond portion of the substrate corresponding to a second feature. Theportion of the substrate corresponding to the second feature is to beremoved. The at least one compensating region of the mask provides themask with the capacity to affect the first portion of the substrate.When the second feature or features are removed from the substrate, whatis left is an isolated feature that has dimensions similar to if thefeature were formed in a group of features.

[0035]FIGS. 2a-c illustrate a stages in an embodiment of a processaccording to the present invention for forming an isolated feature 9 anda similarly sized feature 11, where the feature 11 is arranged in agroup of features. As shown in FIG. 2a, feature 9 is formed in thesubstrate with adjacent features 13 and 15 arranged on either side ofit. Features 13 and 15 are referred to herein as “compensatingfeatures”. The features are formed by corresponding compensating regionsin a microlithography mask.

[0036] Feature 11 is also formed with features 17 and 19 arranged oneither side of it. Unlike features 17 and 19, as illustrated in FIG. 2a,features 13 and 15 are then removed, leaving feature 9 in isolation asillustrated in FIG. 2c. As a result, the substrate includes feature 9 inisolation with a width w_(i) and feature 11 grouped between features 17and 19 with a width w_(n). However, unlike the result illustrated inFIGS. 1a and b, in FIG. 2c, w_(i)=w_(n).

[0037] After forming the features are shown in FIG. 2a, all the featuresmay be developed and etched. A layer of photoresist may then be appliedover all of the features. In the second layer of photoresist, onlyfeatures 13 and 15 may be exposed. Then, the second layer of photoresistmay be developed and features 13 and 15 etched away, thereby leavingonly the original design feature 9.

[0038] The present invention is not limited to including compensatingfeatures on two sides of a feature and ultimately is to be isolated asfeature 9 is surrounded by features 13 and 15. For example, only onefeature 13 or 15 could be formed adjacent feature 9. Additionally, oralternatively, features could be formed adjacent the top and bottom offeature 9, when considering the orientation of the views shown in thefigures. This means the compensating features are in the same plane,just adjacent a different side of feature 9 as compared to compensatingfeatures 13 and 15. Actually, any number of features with any shapecould be formed at any location around the feature such as feature 9that is ultimately to be an isolated feature on the substrate.

[0039] In view of the above, a microlithography mask according to thepresent invention may include at least one compensating region adjacentat a feature to be isolated on a substrate. Therefore, a mask accordingto the present invention could include two compensating regions adjacenteach of two sides of a first region. Such a mask would be utilized in aprocess illustrated in FIGS. 2a-2 c.

[0040] The shape of the regions in the mask could be whatever shape isdesired or necessary to achieve the desired exposure by diffraction ofthe feature that is to be isolated on the substrate. A mask that wouldbe utilized in a process to form features shown in FIGS. 2a-2 c wouldhave rectangular-shaped regions. The compensating regions may extendalong the side of the isolated feature forming region only as far asregions would extend a mask wherein grouped features are to be formed.In any case, preferably, the compensating regions in the mask result insufficient energy being diffracted to the isolated feature portion ofthe substrate to result in an isolated feature being formed that has thesame dimensions as the feature would have if it were formed in a groupof features.

[0041] The second invention may also include a second mask that includesonly at least one compensating region in substantially the same locationas the compensation regions are located in the first mask.

[0042] As stated above, the present invention also includes a method anda mask for mask CD metrology. Mask CD metrology tools have their ownproximity effects. For example, optical microscope-based CD metrologytools are subject to optical diffraction effects. Additionally, scanningelectron microscope-based CD metrology tools are subject to electronscattering. These proximity effects can cause features with identicalphysical size to be measured differently, depending upon whether thefeatures are in an isolated environment or in a nested environment withother features in the vicinity.

[0043] A group of features such as those illustrated in FIGS. 3a-3 b maybe utilized to quantify the proximity effects of CD metrology tools.FIG. 3a illustrates two features 21 and 23 fabricated in the same nestedenvironment. Therefore, features 21 and 23 have substantially similarphysical sizes. Features 25 and 27 are subsequently removed, resultingin features as illustrated in FIG. 3b.

[0044] Features 21 and 23 are then measured with a CD metrology tool.Because of proximity effects of the CD metrology tool, features 21 and23 will not be measured the same even though they may have substantiallythe same physical size. Thus, the difference between the measurements ofw_(i) and w_(n) is a quantitative measurement of the proximity effectsof the CD metrology tool. This difference can be subsequently used tocorrect the CD metrology tool.

[0045] The group of features illustrated in FIGS. 4a and 4 b can beutilized to measure the true CD difference between nested features andisolated features without knowing the exact proximity effect of CDmetrology tools. As shown in FIG. 4a, feature 33 is printed withoutadjacent features, while feature 35 is printed in a nested environment.Therefore, features 33 and 35 have different physical sizes. Features 37and 39 are subsequently removed, resulting in features shown in FIG. 4b.

[0046] Features 33 and 35 are subsequently measured on a CD metrologytool. Because both features 33 and 35 exist in an isolated environment,the proximity effect of the CD metrology is similar. Consequently, themeasurement difference between w_(i) and w_(n) is the truenested-to-isolated CD difference.

[0047] The above-described techniques may also be utilized at the waferlevel to characterize wafer CD metrology tools as well as to measuretrue nested-to-isolated differences of wafer processes.

[0048] The foregoing description of the invention illustrates anddescribes the present invention. Additionally, the disclosure shows anddescribes only the preferred embodiments of the invention, but asaforementioned, it is to be understood that the invention is capable ofuse in various other combinations, modifications, and environments andis capable of changes or modifications within the scope of the inventiveconcept as expressed herein, commensurate with the above teachings,and/or the skill or knowledge of the relevant art. The embodimentsdescribed hereinabove are further intended to explain best modes knownof practicing the invention and to enable others skilled in the art toutilize the invention in such, or other, embodiments and with thevarious modifications required by the particular applications or uses ofthe invention. Accordingly, the description is not intended to limit theinvention to the form disclosed herein. Also, it is intended that theappended claims be construed to include alternative embodiments.

What is claimed is:
 1. A microlithography mask, the mask comprising: afirst region for exposing a first portion of the substrate correspondingto a first feature that is to be formed on the substrate; at least onecompensating region in the vicinity of the first region for partiallyexposing the first portion of the substrate and a second portion of thesubstrate corresponding to a second feature, wherein the second featureis to be removed from the substrate.
 2. The mask according to claim 1,including two compensating regions including a region adjacent each oftwo sides of the first region.
 3. The mask according to claim 1, whereinthe first region is rectangular shaped and wherein the mask includes acompensating region adjacent each of the longer sides of the rectangle.4. The mask according to claim 1, including a plurality of compensatingregions, at least one adjacent each of a plurality of sides of the firstregion.
 5. The mask according to claim 1, wherein the compensatingregion does not extend in a linear direction beyond a side of the firstregion.
 6. The mask according to claim 4, wherein the compensatingregions do not extend in a linear direction beyond the side of the firstregion that they are adjacent.
 7. The mask according to claim 1, whereinthe first feature is to be an isolated feature on the substrate andwherein the compensating region results in exposure of the first featureas if the first feature were in a group of features.
 8. The maskaccording to claim 1, wherein the first feature is to be an isolatedfeature on the substrate and wherein the mask includes a plurality ofcompensating regions arranged about the first region such that thecompensating regions result in exposure of the first feature as if thefirst feature were in a group of features.
 9. The mask according toclaim 1, further comprising: a second mask including the at least onecompensating feature.
 10. The mask according to claim 1, wherein themask produces equal size features in a substrate.
 11. A method offorming an isolated image segment having physical characteristics of anoptical proximity affected segment in a semiconductor mask, the methodcomprising the steps of: exposing a portion of a first photoresist layeron a substrate corresponding to a first feature that is to be formed inthe first photoresist layer; exposing at least one compensating portionof the first photoresist layer adjacent the first region such thatexposing the compensating portion at least partially exposes the firstfeature; and removing the at least one compensating feature.
 12. Themethod according to claim 11, further comprising the steps of:developing the first feature and the at least one compensating feature;applying a second layer of a photoresist at least over the first featureand the at least one compensating feature; exposing the at least onecompensating feature in the second photoresist layer over where the atleast one compensating feature was formed in the first photoresistlayer; developing the at least one compensating feature in the secondphotoresist layer; and etching away the at least one compensatingfeature in the second photoresist layer.
 13. The method according toclaim 12, further comprising the step of: etching the first feature andthe at least one compensating feature prior to applying the secondphotoresist layer.
 14. The method according to claim 12, furthercomprising the steps of: providing a first mask including a first regionfor exposing the first portion of the first photoresist layer and atleast one compensating region in the vicinity of the first region forpartially exposing the first feature and a second portion of the firstphotoresist layer corresponding to the at least one compensatingfeature; and providing a second mask including at least compensatingregion for exposing the at least one compensating feature in the secondphotoresist layer over where the at least one compensating feature wasformed in the first photoresist layer.
 15. The method according to claim14, wherein the first mask includes two compensating regions including aregion adjacent each of two sides of the first region and the secondmask includes two compensating regions at locations corresponding tolocations of the two compensating regions in the first mask.
 16. Themethod according to claim 14, wherein the first region in the first maskis rectangular shaped, the first mask includes a compensating regionadjacent each of the longer sides of the rectangle, and the second maskincludes two compensating regions at locations corresponding tolocations of the two compensating regions in the first mask.
 17. Themethod according to claim 14, wherein the first mask includes aplurality of compensating regions, at least one adjacent each of aplurality of sides of the first region, and the second mask includes twocompensating regions at locations corresponding to locations of the twocompensating regions in the first mask.
 18. The method according toclaim 14, wherein the at least one compensating region does not extendin a linear direction beyond a side of the first region.
 19. The methodaccording to claim 17, wherein the compensating regions do not extend ina linear direction beyond the side of the first region that they areadjacent.
 20. The method according to claim 11, wherein the firstfeature is to be an isolated feature on the substrate and wherein thecompensating region results in exposure of the first feature as if thefirst feature were in a group of features.
 21. The method according toclaim 14, wherein the first feature is to be an isolated feature on thesubstrate and wherein the first mask includes a plurality ofcompensating regions arranged about the first region such that thecompensating regions result in exposure of the first feature as if thefirst feature were in a group of features.
 22. A method for determiningoptical proximity effects of critical dimension metrology tools, themethod comprising the steps of: exposing a first portion of a firstphotoresist layer on a substrate corresponding to a first feature thatis to be formed in the first photoresist layer; exposing at least onecompensating portion of the first photoresist layer adjacent the firstregion to form at least one compensating feature such that exposing thecompensating feature at least partially exposes the first feature;exposing a nested portion of the first layer of photoresistcorresponding to a nested feature that is to be formed in the firstphotoresist layer corresponding to the first portion; exposing a numberof surrounding portions of the first layer of photoresist adjacent thenested portion to form surrounding features such that the number andarrangement of the surrounding features corresponds to the number ofcompensating features and the arrangement of the at least onecompensating feature with respect to the first portion, resulting in thefirst feature and the nested feature having substantially similardimensions; removing the at least one compensating feature; measuringthe first feature and the nested feature with the metrology tool;determining a difference between the measured dimensions of the firstfeature and the nested feature to determine the optical proximityeffects of the metrology tool.
 23. The method according to claim 22,further comprising the step of: correcting the metrology tool bycorrecting for the optical proximity effects determined by determiningthe difference in the measured dimensions of the first feature and thenested feature.
 24. A method for determining a difference in criticaldimension of a nested feature as compared to an isolated feature, themethod comprising the steps of: exposing a first portion of a firstphotoresist layer on a substrate corresponding to a first feature thatis to be formed in the first photoresist layer; exposing a nestedportion of the first layer of photoresist corresponding to a nestedfeature that is to be formed in the first photoresist layercorresponding to the first portion; exposing at least one surroundingportion of the first layer of photoresist adjacent the nested portion toform at least one surrounding feature; removing the at least onesurrounding feature; measuring the first feature and the nested featurewith the metrology tool; and determining a difference between themeasured dimensions of the first feature and the nested feature todetermine a difference in critical dimension of a nested feature ascompared to an isolated feature.